<!DOCTYPE html>
<html>
<body>
<select>
  <option selected>Apple</option>
  <option>Orange</option>
  <option>Banana</option>
</select>
<script>
  var comboBox = document.querySelector('select');
  comboBox.focus();
  // Expand the combo box.
  var clickEvent = document.createEvent('MouseEvents');
  clickEvent.initMouseEvent('mousedown', true, true, window);
  comboBox.dispatchEvent(clickEvent);
  function go() {
    document.querySelector('select').selectedIndex = 1;
  }
</script>
</body>
</html>
